The present invention generally relates to semiconductor devices and more particularly to a semiconductor device in which a semiconductor chip is mounted upon a package substrate.
In a semiconductor device called high frequency module designed for high frequency applications, an MMIC (monolithic microwave integrated circuit) is formed by mounting a compound semiconductors chip such as a GaAs chip upon a mounting substrate such as a module substrate by flip-chip mounting process. With such semiconductor devices of high frequency applications, Au stud bumps of low resistance are formed on a semiconductor chip and the stud bumps are joined to corresponding Au electrode pads on the mounting substrate upon the flip-chip mounting.
Such joining of Au stud bumps to Au electrode pads is achieved by thermal pressure welding, pressure welding that uses adhesive, ultrasonic joining, or the like, wherein a temperature of 300-400° C. is needed in the case of thermal pressure welding.
Conventionally, ceramic substrates were used for the mounting substrate in such semiconductor devices for high frequency applications, while there is a demand in these days to use low-cost resin substrates for the mounting substrate also in such semiconductor devices for high frequency applications. When a resin substrate is used, however, it is not possible to use thermal welding process, which requires the temperature of 300-400° C. as noted before.
In the case the joining is achieved by adhesive, on the other hand, there is no need of heat treatment, and no problem is caused with regard to the heat resistance of the mounting substrate. However, in the case that an Au stud bump and an Au electrode pad are joined by an adhesive, these members are merely contacted with each other, and problems such as increase of resistance or and snapping wire are tend to be caused when shock or thermal cycling is applied.
Under these situations, there is increasing importance of ultrasonic joining technology that can achieve fusion between the Au stud bump and the Au electrode pad at low temperatures.
In order to achieve reliable and strong joining with such ultrasonic joining technology, the surface state of the metal to be joined is very import.